Due to a very small depth of focus, standard photolithography techniques have insufficient fidelity for defining photonic crystal structures on LED epiwafers. But high- quality, large-scale patterning is possible by turning to a novel self-imaging photolithography technique, say Harun Solak, Christian Dais and Francis Clube from Eulitha.

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he introduction of light extraction techniques has spurred an increase in GaN LED efficiency. That’s

because this type of technology can prevent a high proportion of light being trapped and eventually absorbed in the high refractive index semiconductor layers where emission is generated.

One of the most effective light extraction technologies involves etching regular arrays of holes into the emitting surface. Such photonic crystal structures cut the proportion of light propagating within the guided modes of the high index dielectric layer, and channel more emission out of the structure. Indeed, some of the highest performing devices have been created in this way by researchers at manufacturers such as Philips Lumileds and Osram. In addition, there are highly successful commercial products employing this concept, such as the series of Phlatlight LEDs from Luminus Devices.

There are many different ways to incorporate photonic crystal structures into LEDs. Arguably the most straightforward is the etching of a hole structure into the top layer, leaving air holes that can extend into the active region. Other approaches include incorporating a lower index dielectric - such as silicon dioxide - as pillars in GaN, or patterning the substrate with a photonic crystal structure.

Adding a photonic crystal pattern influences light emission in two ways: overall extraction increases by a factor of two-to-three; and the emission profile changes, becoming more concentrated around the surface normal. This enhanced directionality of the emission results in a brighter LED, which is especially important for applications where the light needs to be further guided, collimated or focused. Through careful design of the photonic crystal structure it is possible to tailor the LED’s emission pattern to the target application. Electromagnetic simulation codes are one tool for realizing this. They can determine the effect of parameters such as the period or lattice symmetry.

An alternative, popular practice within the industry for boosting efficiency is roughening or texturing of the LED surface. This introduces facets at different angles, making it easier for light to escape from the chip. One downside of this approach is that it offers no gain or control over the directionality of the emitted light. What’s more, by turning to an optimized and highly controlled photonic crystal pattern instead of a textured surface, it may be possible to increase the process and emission reproducibility across chips and wafers, leading to higher production yields.

Despite the research programs being undertaken by major manufacturers and their published results, the application of photonic crystals to LEDs has not yet been adopted